Cytostatic factor inactivation is induced by a calcium-dependent mechanism present until the second cell cycle in fertilized but not in parthenogenetically activated mouse eggs

Cytostatic factor (CSF) is an activity responsible for the metaphase II arrest in vertebrate oocytes. This activity maintains a high level of maturation promoting factor (MPF) in the oocyte and both activities are destroyed after fertilization or parthenogenetic activation. To study some of the characteristics of the mechanism involved in MPF and CSF destruction, we constructed hybrid cells between metaphase II arrested oocytes and early embryos obtained after fertilization or artificial activation. We found that the behavior of hybrid cells differed depending upon the type of oocyte activation. Initially, the reaction of both types of hybrid cells was similar, the nuclear envelope broke down and chromatin condensation was induced. However, while metaphase II oocytes fused with parthenogenetic eggs remained arrested in M-phase, the oocytes fused with fertilized eggs underwent activation and passed into interphase. This ability of fertilized eggs to induce oocyte activation was still present at the beginning, but not at the end of the second embryonic cell cycle. Oocyte activation induced by fusion with a fertilized egg could be prevented when calcium was chelated by BAPTA. Thus, element(s) of the mechanism involved in calcium release triggered by a sperm component at fertilization remain(s) active until the second cell cycle and is (are) inactivated before the end of the 2-cell stage.